1. Field of the Invention
The present invention relates to a liquid crystal display and a method of manufacturing the same and, more particularly, to a transflective liquid crystal display capable of display in both reflective and transmissive modes and a method of manufacturing the same.
2. Description of the Related Art
Among active matrix type liquid crystal displays, attention is recently paid to reflective liquid crystal displays which can be provided with a light weight and a small thickness and which consume low power. Reflective liquid crystal displays that are currently in practical use are one-polarizer type displays utilizing a TN (Twisted Nematic) mode liquid crystal (see Patent Documents 1 and 2 for example). However, a one-polarizer type reflective liquid crystal display has a problem in that it undergoes a significant reduction in visibility in an environment of low brightness because the visibility of the display significantly depends on the brightness of the surroundings.
Meanwhile, a transmissive liquid crystal display is characterized in that it exhibits high contrast and visibility even in a dark environment although it consumes high power because of the use a backlight unit as a light source. However, a transmissive liquid crystal display has a problem in that it undergoes a significant reduction in visibility and therefore becomes poorer in display quality than a reflective liquid crystal display in a bright environment.
Techniques for mitigating the above-described problems of the reflective and transmissive liquid crystal displays include front-light type liquid crystal displays which are combinations of a reflective liquid crystal display and a front-light unit and transflective liquid crystal displays utilizing transflective films as pixel electrodes (see Patent Document 3 for example). However, a front-light type liquid crystal display has a problem in that it has a contrast ratio lower than that of a transmissive liquid crystal display in a dark environment and in that it displays an object darker than a normal reflective liquid crystal display does in a bright environment due to absorption of light at a light guide plate of the front-light unit.
On the contrary, Metal thin films such as aluminum (Al) thin films having a thickness of about 30 nm are normally used as transflective films of a transflective liquid crystal display as described above. A metal thin film has a problem in that it reduces utilization of light because it has a great light absorption coefficient. Further, since it is very difficult to form a metal thin film having a great area and a uniform thickness, a problem arises in that a variation of the thickness of a transflective film can result in a great in-plane variation of the transmittance of the film.
Techniques for solving the above-described problems include transflective liquid crystal displays which have a reflective area formed with a reflective electrode reflecting light and a transmissive area formed with a transparent electrode transmitting light at each pixel (see Patent Document 4 for example). Such a transflective liquid crystal display can display an object with a relatively high contrast ratio in both of the reflective and transmissive modes, and it has no in-plane variation of transmittance.
In a transflective liquid crystal display, however, light passes through a liquid crystal layer only once during display in the transmissive mode, whereas light passes through the liquid crystal layer twice during display in the reflective mode. In the configuration disclosed in Patent Document 4, since a transmissive area and a reflective area have substantially the same cell thickness (the thickness of the liquid crystal layer) and liquid crystal alignment, substantial retardation in the reflective area is approximately twice retardation in the transmissive area when consideration is paid to the fact that light passes through the liquid crystal layer twice in the reflective area. When the cell thickness is set such that display is preferably performed in either of the reflective and transmissive modes, the brightness and contrast ratio are reduced in the other mode. A problem therefore arises in that it is not possible to perform preferable display in both of the reflective and transmissive modes.
Techniques for solving the above-described problem include dual cell gap type transflective liquid crystal displays in which a reflective area and a transmissive area have different cell thicknesses (see Patent Document 5 for example). FIG. 26 shows a sectional configuration of such a transflective liquid crystal display. FIG. 26 shows a transmissive area T of a pixel region on the left side thereof and a reflective area R on the right side thereof. As shown in FIG. 26, the transflective liquid crystal display has a thin film transistor (TFT) substrate 102 and an opposite substrate 104 which are provided opposite to each other and a liquid crystal 106 sealed between the substrates 102 and 104. A pair of polarizers 186 and 187 is provided so as to sandwich the substrates 102 and 104. The TFT substrate 102 has a transparent pixel electrode 116 formed on a glass substrate 110. A leveling film 134 having a thickness of about 2 μm is formed on the pixel electrode 116 in the reflective area R to make a cell thickness dR in the reflective area R smaller than a cell thickness dT in the transmissive area T. A reflective electrode 117 is formed on the leveling film 134. The opposite substrate 104 has a transparent common electrode 142 formed on a glass substrate 111.
Since a voltage applied to the liquid crystal 106 is substantially constant in the same pixel, liquid crystal molecules 108T in the transmissive area T and liquid crystal molecules 108R in the reflective area R are tilted at substantially the same tilting angle. Therefore, refractive index anisotropy ΔnT of the transmissive area T and refractive index anisotropy ΔnR of the reflective area R are substantially equal to each other (ΔnT≈ΔnR). The cell thickness dR in the reflective area R is smaller than the cell thickness dT in the transmissive area T (dT>dR), and the cell thickness dR is about one-half of the cell thickness dT (dT≈2·dR). Thus, the substantial retardation in the reflective area R and the retardation in the transmissive area T are substantially equal to each other, and sufficient brightness and contrast ratio can therefore be achieved in both of the reflective and transmissive modes.
However, since there is a need for forming the leveling film 134 in the reflective area R, the transflective liquid crystal display has a problem in that it involves complicated manufacturing processes which results in an increase in the manufacturing cost. In the transflective liquid crystal display, since the reflective area R and the transmissive area T have different cell thicknesses, the speed of response of the liquid crystal 106 varies accordingly. Further, since the thickness of the leveling film 134 is about one-half of the cell thickness in the transmissive area T, a relatively large step is formed at a boundary between the reflective area R and the transmissive area T. Such a step can cause a defect in the alignment of the liquid crystal 106 and can accumulate a liquid applied to form an alignment film.
Let us assume that spherical spacers are used to maintain the cell thicknesses of the above-described transflective liquid crystal display. Then, if the diameter of the spherical spacers is adjusted to the cell thickness of either of the reflective area R and the transmissive area T, the desired cell thickness can not be achieved in the other area when the spherical spacers are disposed in the other area. As thus described, a dual gap type transflective liquid crystal display has a problem in that it is difficult to control cell thicknesses of the same.
Another problem arises in that a difference between light paths in the reflective area R and the transmissive area T results in a difference in color purity between the reflective and transmissive modes. Specifically, light passes through a CF layer once in the transmissive area T, whereas light passes through a CF layer twice and therefore has a relatively low transmittance in the reflective area R. When the color purity of the CF layer is adjusted to perform display with high brightness in the reflective mode, the color purity of display in the transmissive mode becomes quite low to render the display too light. When the color purity of the CF layer is adjusted to perform preferable display in the transmissive mode, transmittance becomes low during display in the reflective mode to render the display very dark. Patent Document 6 discloses a technique for providing a reflective area R and a transmissive area T with different degrees of color purity to mitigate the problem. According to the technique, a CF layer is provided throughout the transmissive area T, whereas the CF layer is formed in parts of the reflective area R to correct the color purity of the area. According to this technique, however, there is a need for a new processing step for leveling steps which are produced by the partially formed CF layer. This necessitates a greater number of manufacturing processes than a step for fabricating an opposite substrate 104 having an ordinary CF layer and consequently results in an increase in the manufacturing cost of a liquid crystal display.
Patent Document 1: Japanese Patent Laid-Open No. JP-A-5-232465
Patent Document 2: Japanese Patent Laid-Open No. JP-A-8-338993
Patent Document 3: Japanese Patent Laid-Open No. JP-A-7-333598
Patent Document 4: Japanese Patent Laid-Open No. JP-A-11-281972
Patent Document 5: Japanese Patent No. 3380482
Patent Document 6: Japanese Patent No. 3410664
Patent Document 7: Japanese Patent Laid-Open No. JP-A-2002-296585